1. Field of the Invention
The invention relates to the field of electromagnetic shielding via conductive gaskets that bridge across openings between parts of conductive housings. According to the invention the shielding structure is defined by a plurality of resilient conductively wrapped segments, electrically coupled together by conductive tape or the like to form a gasket. The invention is particularly useful for shielding relatively large areas such as bypass panels at which input/output lines traverse the boundary of a shielded enclosure, typically via connectors. The segmented structure is such that the conductive path from a given point on the panel to a given point on the housing or other ground point, is shorter and/or lower in resistance than in a non-segmented arrangement, which improves shielding efficiency as compared to a similar gasket that is continuous rather than segmented.
2. Prior Art
To reduce problems due to alternating electromagnetic fields emanating from electronic equipment and/or due to the tendency for incident electromagnetic fields to affect sensitive circuits, a conductive barrier or shield is placed along the path of field propagation to provide a discontinuity. The conductive barrier typically is coupled electrically to a circuit ground. Part of the electromagnetic energy incident on the shield is reflected, and part of the energy induces currents in the shield. These currents are dissipated as eddy currents. The field is attenuated by the shield.
Conventional shielding typically uses the external housing of an article of electronic equipment for at least part of the shield barrier. In addition, internal shield barriers, enclosures, ground planes and the like can be provided for particular circuit elements within the enclosure. For these purposes, sheet metal materials, laminates of metal and plastic and/or conductive coatings typically form the conductive electromagnetic shield barrier.
Many modern electronic devices emit or are sensitive to electromagnetic interference or "EMI" at high frequencies. For example, computer clock and digital data signals, phase locked loops, switched mode power supplies, various radio frequency and microwave devices and the like are sources of EMI. Many such electronic circuits also are susceptible to EMI, and must be shielded in order to operate properly.
The effectiveness of a shield is a function of a number of factors including the electrical properties of the shield material (e.g., conductivity and magnetic permeability), the thickness and continuous or discontinuous nature of the shield, the frequency of the EMI, the spacing and configurations of the EMI source and the shield, etc. For shielding relatively higher frequencies, any gaps in the conductive material must be smaller to prevent leakage as compared to shielding lower frequencies.
The frequencies to be attenuated include harmonics. For a computer, for example, having a basic clock frequency of 25 to 100 MHz, significant harmonics may be present up to 900 MHz or more. Effective shielding requires a nearly continuous (i.e., gapless) shield, preferably arranged close to the source of EMI and/or close to the susceptible circuit, and made of a highly conductive material.
Typically, shielding is provided by a conductive enclosure made of thin sheet metal, metallized plastic or the like. An external housing can form a conductive shielding enclosure, and shielding subenclosures can be provided for subassemblies within the housing. Effective shielding advantageously includes conductive EMI blocking gaskets that continue the conductive barrier of a shield across any gaps or seams between conductive panels, enclosures, doors, housing elements and the like, which form portions of the shield.
Resiliently compressible conductive gasket structures are disclosed, for example, in U.S. Pat. Nos. 4,857,668--Buonanno; 5,045,635--Kaplo et al.; and 5,202,536--Buonanno, which are hereby incorporated. According to these patents, a conductive sheet material such as a woven or unwoven metalized plastic fabric is provided on a resilient compressible core of indefinite length to form an elongated linear gasket that can be placed between conductive panels to bridge any gap between them, e.g., due to discontinuities and the like. One possible conductive material is Monsanto "Flectron" nickel/copper metallized polyester fabric.
A conductive gasket as described can be formed in various cross sectional shapes, including round, rectangular and irregular shapes, and can be provided with an attachment means such as a clip or an adhesive area, to assist in mounting. Conductive gaskets can be made to the specific shape and area needed to reside between the conductive surfaces between which they are to seal, but it is generally more difficult and expensive to provide customized shapes and sizes. In addition, conductive gaskets for large sealing areas are more difficult to make than small area gaskets. As a result, relatively slender linear conductive gaskets or seals are generally mounted so as to define lines electrically connecting between conductive panels. Where an opening is to be provided, the linear seals are arranged around the perimeter of the opening.
For a conductive gasket comprising a conductive sheet on a resilient core, the core can be molded, extruded, cut from a block of resilient stock, etc. The conductive sheet can be wrapped on the core and affixed, for example, by adhesive. According to the above Buonanno patents, the core also can comprise a polymer with a foaming agent, applied as a liquid to a woven or non-woven metallized fabric that is wrapped, for example into a closed shape. The polymer expands to fill the void and as the polymer cures, it engages securely with the fabric.
Providing and wrapping a resilient core can be accomplished readily for substantially linear gaskets in round, oval, rectangular or complex cross sectional shapes that cover a small surface area. Where the surface area is larger, and particularly where the thickness between opposite surfaces is minimal, it may be difficult to form and/or wrap the gasket in a manner that provides an accurate shape and a smooth conductive covering. On the other hand, slender linear gaskets are not suitable for all EMI sealing applications, for example, where a substantial area of abutment between housing elements is to be sealed and/or when the sealed surface is discontinuous, for example, to allow clearance for connectors.
For larger areas, it would be possible to provide a pattern of linear seals to seal between the surfaces. Linear seals could be arranged around the perimeters of openings for connectors and the like, to allow clearance while sealing between the conductive panels. But this is cumbersome. A customized wide area gasket with clearance openings is possible, but this may be cumbersome and expensive to produce.
Another difficulty with wide area gaskets is that assuming the compressible core is not conductive, then the shortest conductive path from any point on one side of the gasket to any point on the opposite side must pass clear across the surfaces on opposite sides of the gasket to the edge. Therefore, wide gaskets may be characterized by a relatively higher point-to-point resistance and reduced EMI shielding efficiency as compared to a plurality of individual elongated seals. On the other hand, individual seals are more complicated to mount.
What is needed is an improved means for sealing wide areas, having the ease of manufacture of a simple elongated linear seal of preferably regular shape, and the versatility and wide area coverage of a more customized shape. Advantageously, such a gasket would also have a low resistivity.
According to the present invention, a large area EMI sealing gasket is provided in the form of a sheet, made by attaching a plurality of laterally adjacent elongated segments using conductive tape or the like. The sheet can be die cut to size and to provide clearance openings, including openings larger than a segment width. Moreover, whereas the segments as thereby connected provide additional and shorter paths between the sealed surfaces than a comparably sized continuous seal or gasket, the shielding efficiency of the shield is in fact improved rather than diminished, due to its segmented form.